Grease

ABSTRACT

The present invention provides a grease showing low thermal resistance, which is not significantly deteriorated by a heat cycle, particularly, a grease suitable as a thermally conductive material for heat-generating electronic components. 
     A grease containing one type or at least two types of thermally conductive material powder selected from the group consisting of a thermally conductive material (A), a thermally conductive material (B) and a thermally conductive material (C), wherein the particle size distribution of the thermally conductive material powder measured by a laser diffraction type particle size distribution method, has population peaks in the ranges of from 2.0 to 10 μm, from 1.0 to 1.9 μm and from 0.1 to 0.9 μm, respectively, and the grease contains a base oil having a surface tension of from 25 to 40 dyn/cm at 25° C.

TECHNICAL FIELD

The present invention relates to a thermally conductive grease.

BACKGROUND ART

Along with size reduction and high output power of heat-generating electronic components such as CPUs (central processing units) of personal computers, heat amount generated from a unit area of these electronic components has become extremely large. The total heat amount of these electronic components corresponds to about 20 times of that of an iron for pressing clothes. In order to prevent trouble of these heat-generating electronic components for a long time, cooling of these heat-generating electronic components is necessary. For cooling, a heat sink or a housing made of metal is employed, and further, in order to efficiently transfer a heat from a heat-generating electronic component to a cooling portion such as a heat sink or a housing, a thermally conductive material is employed. The reason for using such a thermally conductive material, is that if such a heat-generating electronic component is directly contact with e.g. a heat sink, air is present on their interface from microscopic observation, which prevents heat conduction. Accordingly, instead of the air present on the interface, by disposing a thermally conductive material in the gap between the heat-generating electronic component and e.g. the heat sink, it is possible to efficiently transfer heat.

As such a thermally conductive material, a thermally conductive sheet produced by curing a silicone rubber blended with a thermally conductive powder; a thermally conductive pad that has flexibility and produced by curing a flexible silicone such as a silicone gel blended with a thermally conductive powder; a thermally conductive grease having flowability produced by blending a liquid silicone with a thermally conductive powder; or a phase change type thermally conductive material which becomes soft or flowable according to the operation temperature of a heat-generating electronic component; may be mentioned. Among these, a thermally conductive grease is particularly suitable for transferring heat.

A thermally conductive grease is produced by blending a base oil that is a liquid silicone such as a silicone oil with a thermally conductive powder. In order to satisfy requirement for high thermal conductivity, it is proposed to employ an aluminum nitride powder as the thermally conductive powder (Patent Document 1). However, since an aluminum nitride powder has a hexagonal crystal structure and its particle shape is not spherical, it is not possible to sufficiently increase packing amount of such a thermally conductive powder to achieve sufficiently high thermal conductivity.

When a base oil being a dimethyl silicone oil is blended with an alumina powder and an aluminum nitride powder (Patent Documents 2 and 3) or with an alumina powder and a metal aluminum powder (Patent Document 4) and used as a thermally conductive grease, the grease shows high thermal conductivity, but when it receives a heat cycle between low temperature and high temperature repeatedly for a long period of time, so-called “oil separation” that is separation of silicone oil being a base oil, occurs to increase thermal resistance.

Meanwhile, in order to solve the problem of separation of silicone oil being a base oil, it is proposed to use a special silicone (Patent Document 5), but this document does not describes as to high thermal conductivity.

Patent Document 1: JP-A-2000-169873

Patent Document 2: JP-A-2002-194379

Patent Document 3: JP-A-2005-54099

Patent Document 4: JP-A-2005-170971

Patent Document 5: JP-A-2004-917743

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is an object of the present invention to provide a grease showing low thermal resistance and improved durability against heat cycle, in particular to provide a grease suitable as a thermally conductive material for heat-generating electronic components.

Means for Solving the Problems

In order to solve the above problems, the present invention employs the following means.

(1) A grease containing one type or at least two types of thermally conductive material powder selected from the group consisting of a thermally conductive material (A), a thermally conductive material (B) and a thermally conductive material (C), wherein the particle size distribution of the thermally conductive material powder measured by a laser diffraction type particle size distribution method, has population peaks in the ranges of from 2.0 to 10 μm, from 1.0 to 1.9 μm and from 0.1 to 0.9 μm respectively, and the grease contains a base oil having a surface tension of from 25 to 40 dyn/cm at 25° C. (2) A grease containing a thermally conductive material (A) having an average particle size of from 2.0 to 10 μm, a thermally conductive material (B) having an average particle size of from 1.0 to 1.9 μm, a thermally conductive material (C) having an average particle size of from 0.1 to 0.9 μm, and a base oil having a surface tension of from 25 to 40 dyn/cm at 25° C. (3) The grease according to the above (1) or (2), wherein the thermally conductive material (A), (B) or (C) is one type or at least two types selected from the group consisting of metal aluminum, aluminum nitride and zinc oxide. (4) The grease according to the above (1) or (2), wherein the thermally conductive material (A) is metal aluminum, the thermally conductive material (B) is aluminum nitride, and the thermally conductive material (C) is zinc oxide. (5) The grease according to any one of the above (1) or (4), wherein the viscosity of the base oil is from 300 to 1,000 mPa·s. (6) The grease according to any one of the above (1) or (5), wherein the base oil is a silicone oil modified to have specific alkyl group. (7) The grease according to any one of the above (1) or (6), wherein the content of the thermally conducive materials (A), (B) and (C) is from 60 to 80 vol %. (8) The grease according to any one of the above (1) or (7), wherein based on the total thermally conductive material, the thermally conductive material (A) is from 50 to 70 vol %, the thermally conductive material (B) is from 30 to 20 vol % and the thermally conductive material (C) is from 20 to 10 vol %. (9) The grease according to any one of the above (1) or (8), which further contains a silane coupling agent. (10) The grease according to any one of the above (1) or (9), which has a thermal resistance of at most 0.2° C./W.

EFFECTS OF THE INVENTION

The present invention provides a grease suitable for thermal conduction of e.g. heat generated from electronic components. The grease shows low thermal resistance and improved durability against heat cycle.

BEST MODE FOR CARRYING OUT THE INVENTION

The thermally conductive materials (A), (B) or (C) contained in the grease of the present invention is one type or at least two types selected from the group consisting of metal aluminum, aluminum nitride and zinc oxide. The thermally conductive material (A), (B) or (C) may contain a thermally conductive powder such as metal tin, metal silver, metal copper, silicon carbide, aluminum oxide, silicon nitride or boron nitride powder, and such a thermally conductive powder may replace preferably at most 5 vol %, particularly preferably at most 3 vol % of the total amount of metal aluminum, aluminum nitride and zinc oxide for use.

The grease of the present invention contains the powder of thermally conductive material wherein the particle size distribution of the powder measured by a laser diffraction type particle distribution method has population peaks in the ranges of from 2.0 to 10 μm, from 1.0 to 1.9 μm and from 0.1 to 0.9 μm, respectively, whereby the number of contact points between thermally conductive materials can be increased. As a result, it is possible to improve thermal conductivity of the grease. One method of employing a powder of thermally conductive material having a particle distribution having such population peaks, may be a method of blending thermally conductive materials having different particle size distributions.

By blending three types of thermally conductive materials that are the thermally conductive materials (A), (B) and (C) having different average particle sizes, it is possible to increase packing density. Namely, by blending a thermally conductive material (A) having an average particle size of from 2.0 to 10 μm, a thermally conductive material (B) having an average particle size of from 1.0 to 1.9 μm and a thermally conductive material (C) having an average particle size of from 0.1 to 0.9 μm, it is possible to increase the packing density of thermally conductive material. As a result, it is possible to improve the thermal conductivity of grease. Further, by making the grease contain a thermally conductive material having a small average particle size of preferably from 0.1 to 10 μm, more preferably from 0.3 to 6 μm, it becomes possible to reduce the thickness of grease film containing the thermally conductive material, and to reduce the thermal resistance (difficulty of transferring heat). As a result, it becomes possible to produce a grease which extremely easily transfer heat.

The thermally conductive material (A) having an average particle size of from 2.0 to 10 μm to be used in the present invention, needs to have an average particle size of from 2.0 to 10 μm, preferably from 3 to 6 μm. If the average particle size is larger than 10 μm, forming of thin film of grease tends to be difficult, and the thermal resistance of the grease tends to increase. On the other hand, if the average particle size is smaller than 2.0 μm, metal aluminum becomes preferred as the thermally conductive material (A).

The thermally conductive material (B) having an average particle size of from 1.0 to 1.9 μm to be employed in the present invention, needs to have an average particle size of from 1.0 to 1.9 μm, preferably from 1.3 to 1.7 μm. If the average particle size is larger than 1.9 μm, the particle size becomes close to that of the thermally conductive material having an average particle size of from 2.0 to 10 μm, the packing density tends to be poor and the thermal resistance tends to increase. On the other hand, if the average particle size is smaller than 1 μm, the particle size become close to that of the thermally conductive material having an average particle size of from 0.1 to 0.9 μm, the packing density of the thermally conductive material tends to be poor and the thermal resistance tends to increase. The thermally conductive material (B) is preferably aluminum nitride.

The thermally conductive material (C) having an average particle size of from 0.1 to 0.9 μm being a zinc oxide powder to be employed in the present invention, needs to have an average particle size of from 0.1 to 0.9 μm, preferably from 0.3 to 0.7 μm. If the average particle size is larger than 0.9 μm, the particle size becomes close to that of the thermally conductive material having an average particle size of from 1.0 to 1.9 μm, whereby the packing density tends to be poor and the thermal resistance tends to increase. If the average particle size is smaller than 0.1 μm, the packing density of entire thermally conductive material tends to be poor and the thermal resistance tends to increase. The thermally conductive material (C) is preferably zinc oxide.

The content of the thermally conductive materials (A), (B) and (C) in the grease is preferably from 60 to 80 vol %, more preferably from 65 to 75 vol %. If the content of the thermally conductive material exceeds 80 vol %, the grease tends to be hard, and the thermal resistance tends to increase. Further, if the content of the thermally conductive material is less than 60 vol %, since the packing amount of the thermally conductive material is small, heat tends to be hardly transferred, and the thermal resistance tends to increase.

The composition ratio of the three types of thermally conductive materials having different average particle sizes, is such that the thermally conductive material (A) is preferably from 50 to 70 vol %, particular preferably from 55 to 65 vol %, the thermally conductive material (B) is preferably from 30 to 20 vol %, particularly preferably from 27 to 25 vol %, and the thermally conductive material (C) is preferably from 20 to 10 vol %, particularly preferably from 17 to 13 vol %. If the content ratio of the thermally conductive material (A) becomes less than 50 vol %, the grease tends to be hard, and the thermal resistance tends to increase. If it becomes more than 70 vol %, the packing density of the thermally conductive material tends to be poor and the thermal resistance tends to increase.

The average particle size in the present invention was measured by using a “Laser Diffraction Particle Size Analyzer SALD-200” manufactured by Shimadzu Corporation. An evaluation sample was prepared in the following manner. 5 g of thermally conductive powder to be analyzed was added to 50 cc of purified water in a glass beaker, they were stirred by using a spatula, and subjected to dispersion treatment by an ultrasonic cleaner for 10 minutes. The solution of the powder of thermally conductive material subjected to the dispersion treatment, was dropped onto the sampler unit of the analyzer by using a dropping piplet, and left until it became stable enough to measure the light absorptivity. When the light-absorptivity is thus stabilized, the measurement is carried out. The laser diffraction particle size analyzer calculates a particle size distribution from light intensity distribution of light diffracted/scattered by particles detected by a sensor. The average particle size is calculated by multiplying the value of particle size by a relative particle amount (difference %) and dividing the multiplied product by the total 100% of the relative particle amount. Here, the average particle size is the average diameter of particles.

The base oil employed in the present invention has a surface tension at 25° C. of from 25 to 40 dyn/cm, particularly preferably from 30 to 35 dyn/cm. If the surface tension is lower than 25 dyn/cm, when a heat cycle is repeatedly applied to the grease, the base oil tends to be separated, and thereby the grease tends to be hard and the thermal conductivity tends to be poor. Further, if the surface tension is higher than 40 dyn/cm, the wet property of the grease tends to be poor, and the grease is hardly spread and the thermal conductivity tends to be poor.

The surface tension is a property of liquid that the liquid tends to make its surface area as small as possible, and the surface tension is a type of interface tension. When a liquid is in contact with a gas, the liquid has a property of shrinking its surface area as much as possible. A molecule in the liquid is attracted by molecules around the molecule by an attracting force, while a molecule on a surface is not receive an effect of attracting force of other liquid molecules in a direction where the molecule does not contact with the liquid. By this effect, molecules on the surface have excess energy, which produces a surface tension. When the surface tension is strong, the base oil is hard to be separated from the grease.

In the present invention, Wilhelmy method is preferred as a method for measuring surface tension. Wilhelmy method is such that when a plate (mainly a platinum plate) is vertically dipped into a liquid surface, the liquid creeps up along the plate, and increase of the surface area of the liquid produced by the creep up, a surface tension occurs. By dividing the force by the perimeter length (twice of the total of width and thickness) of the plate, a force per a until length (dyne/cm) is obtained. A surface tension can be obtained from this value. As a measurement apparatus of surface force, an “automatic surface tension meter” manufactured by Kyowa Interface Science Co., Ltd., etc. may be employed.

Surface tension of the base oil can be adjusted by adding an additive having a large surface tension to a base oil having small surface tension. For example, by adding a silane coupling agent having an alkyl group to e.g. a dimethyl silicone oil having small surface tension, it is possible to adjust the surface tension.

The viscosity of the base oil is preferably from 300 to 1,000 mPa·s, particularly preferably from 500 to 700 mPa·s. If the viscosity of base oil is less than 300 mPa·s, the base oil and the thermally conductive material in the grease tends to be separated by a heat cycle, whereby the thermal resistance tends to increase. If the viscosity of base oil exceeds 1,000 mPa·s, it tends to be difficult to densely pack the thermally conductive material, whereby the thermal conductivity of the grease tends to be poor.

The viscosity of the base oil is measured by a “Digital Viscosity Meter DV-I” manufactured by Brookfield. Using an RV spindle set, a rotor No. 1 and a container capable of adapting the rotor and capable of containing the base oil to a baseline, are used. The rotor is immersed into the base oil, and the viscosity of the base oil was evaluated at a rotation speed of 10 rpm.

In the present invention, it is preferred to employ as the base oil a silicone oil having a surface tension of from 27 to 37 dyn/cm and a viscosity of from 400 to 800 mPa·s that is produced by modifying a dimethyl silicone oil having a surface tension of preferably from 25 to 40 dyn/cm and having a viscosity of from 300 to 1,000 mPa·s so that its methyl group is modified to have an alkyl group having at least 3, particularly preferably from 8 to 12 carbon atoms. Such a silicone oil modified to have specific alkyl group has large surface tension, and suppresses deterioration of grease in thermal resistance due to heat cycle.

The grease of the present invention contains a silane coupling agent that functions as a surface adulterant which can impart hydrophobic property to the filler, improve its dispersibility and modify the quality of organic resin. A preferred silane coupling agent may, for example, be an alkylsilane having an alkyl group containing from 8 to 10 carbon atoms. An example of preferred silane coupling agent may, for example, be n-octyl trimethoxysilane, n-octyl triethoxysilane or n-decyl trimethoxysilane.

Here, the grease of the present invention may contain besides the above components, an antioxidant or an anticorrosive agent for metals as the case requires.

The grease of the present invention can be produced by kneading the above material by a multi-functional mixer stirrer, a kneader, a hybrid mixer, etc.

The method for measuring the thermal resistance of grease is as follows. A grease is sandwiched between a copper jig of cubic shape in which a heater is embedded and having a leading edge of 1 cm² (1 cm×1 cm) and a copper jig of cubic shape to which a cooling fin is attached and having a leading edge of 1 cm² (1 cm×1 cm), and a load of 4 kg per 1 cm square is applied so that the sample and the copper jigs are closely contact. The amount of sample is selected so that the sample fills the entire contact surface. They are maintained for 30 minutes while 20 W of electric power is applied to the heater, the temperature difference (° C.) between the copper jigs is measured, and the thermal resistance is calculated by using a formula that thermal resistance (° C./W)={temperature difference (° C.)/electric power (W)}.

The thermal resistance of the grease of the present invention is preferably at most 0.2° C./W, particularly preferably at most 0.1° C./W considering the thermal conductivity of grease.

The separation state of the grease of the present invention was evaluated in the following manner. Namely, between two transparent glass plates of 1 mm thick and 10,000 mm² (100 mm×100 mm) area, a grease of 100 μm thick and 900 mm² (30 mm×30 mm) area was applied, and in this state, a heat cycle at −40° C. for 30 min and at 130° C. for 30 min was applied. The number of cycles was 100 cycles. The weight of the base oil separated from the thermally conductive grease was measured to evaluate separation.

EXAMPLES Examples 1 to 24 and Comparative Examples 1 to 8

Thermally conductive materials (A), (B) and (C) shown in Table 1, a base oil (D) shown in Table 2 and a silane coupling agent (E) shown in Table 3, were blended at a ratio shown in Tables 4 to 6, and they were blended for 5 minutes by using “Awatori Rentaro AR-250” manufactured by Thinky, to prepare a grease. The thermal resistance and the separation state of the grease obtained was evaluated and the results are shown in Table 4. Further, in the evaluation result, a thermally conductive grease having a thermal resistance of more than 0.2° C./W was designated as a Comparative Example since it does not transfer heat from a heat-generating portion to a cooling portion efficiently.

TABLE 1 Average particle size No. Type Shape (μm) A-1 Metal aluminum powder Irregular 2.0 A-2 Metal aluminum powder Irregular 5.5 A-3 Metal aluminum powder Irregular 10 A-4 Metal aluminum powder Irregular 0.05 A-5 Metal aluminum powder Irregular 30 A-6 Metal aluminum powder Sphere 3.0 A-7 Metal aluminum powder Irregular 6.0 B-1 Aluminum nitride powder Irregular 1.0 B-2 Aluminum nitride powder Irregular 1.6 B-3 Aluminum nitride powder Irregular 1.9 B-4 Aluminum nitride powder Irregular 0.05 B-5 Aluminum nitride powder Irregular 30 B-6 Aluminum nitride powder Irregular 1.3 B-7 Aluminum nitride powder Irregular 1.7 C-1 Zinc oxide powder Irregular 0.1 C-2 Zinc oxide powder Irregular 0.6 C-3 Zinc oxide powder Irregular 0.9 C-4 Zinc oxide powder Irregular 0.05 C-5 Zinc oxide powder Irregular 30 C-6 Zinc oxide powder Irregular 0.3 C-7 Zinc oxide powder Irregular 0.7

TABLE 2 Surface tension Viscosity No. Type (dyn/cm) (mPa · s) D-1 Alkyl modified 25 500 silicone oil D-2 Alkyl modified 30 550 silicone oil D-3 Alkyl modified 35 600 silicone oil D-4 Alkyl modified 40 1000 silicone oil D-5 Alkyl modified 45 2000 silicone oil D-6 Dimethyl silicone oil 20 100 D-7 Alkyl modified 30 300 silicone oil D-8 Alkyl modified 30 700 silicone oil D-9 Alkyl modified 30 1000 silicone oil D-10 Alkyl modified 30 200 silicone oil D-11 Alkyl modified 30 1200 silicone oil

TABLE 3 No. Type E-1 n-Decyl trimethoxysilane

TABLE 4 Silane Thermally conductive coupling Thermal material (vol %) Base oil agent resistance Content of each material Total (vol %) (vol %) (° C./W) Separation Ex. 1 32.5 19.5 13 65 35 — 0.09 0 g (A-1) (B-2) (C-2) (D-1) Ex. 2 32.5 19.5 13 65 35 — 0.07 0 g (A-2) (B-2) (C-2) (D-1) Ex. 3 32.5 19.5 13 65 35 — 0.09 0 g (A-3) (B-2) (C-2) (D-1) Ex. 4 32.5 19.5 13 65 35 — 0.10 0 g (A-2) (B-1) (C-2) (D-1) Ex. 5 32.5 19.5 13 65 35 — 0.09 0 g (A-2) (B-3) (C-1) (D-1) Ex. 6 32.5 19.5 13 65 35 — 0.09 0 g (A-2) (B-2) (C-3) (D-1) Ex. 7 32.5 19.5 13 65 35 — 0.06 0 g (A-2) (B-2) (C-2) (D-2) Ex. 8 32.5 19.5 13 65 35 — 0.06 0 g (A-2) (B-2) (C-2) (D-3) Ex. 9 32.5 19.5 13 65 35 — 0.08 0 g (A-2) (B-2) (C-2) (D-4) Ex. 10 32.5 19.5 13 65 35 — 0.07 0 g (A-2) (B-2) (C-2) (D-7) Ex. 11 32.5 19.5 13 65 35 — 0.07 0 g (A-2) (B-2) (C-2) (D-8) Ex. 12 32.5 19.5 13 65 35 — 0.07 0 g (A-2) (B-2) (C-2) (D-9)

TABLE 5 Silane Thermally conductive coupling Thermal material (vol %) Base oil agent resistance Content of each material Total (vol %) (vol %) (° C./W) Separation Ex. 13 30 18 12 60 40 — 0.09 0 g (A-2) (B-2) (C-2) (D-1) Ex. 14 40 24 16 80 20 — 0.07 0 g (A-2) (B-2) (C-2) (D-1) Ex. 15 32.5 19.5 13 65 35 — 0.07 0 g (A-6) (B-2) (C-2) (D-1) Ex. 16 32.5 19.5 13 65 35 — 0.07 0 g (A-7) (B-2) (C-2) (D-1) Ex. 17 32.5 19.5 13 65 35 — 0.06 0 g (A-2) (B-6) (C-2) (D-1) Ex. 18 32.5 19.5 13 65 35 — 0.06 0 g (A-2) (B-7) (C-2) (D-1) Ex. 19 32.5 19.5 13 65 35 — 0.06 0 g (A-2) (B-2) (C-6) (D-1) Ex. 20 32.5 19.5 13 65 35 — 0.06 0 g (A-2) (B-2) (C-7) (D-1) Ex. 21 45.5 13  6.5 65 35 — 0.08 0 g (A-2) (B-2) (C-2) (D-1) Ex. 22 32.5 19.5 13 65 35 — 0.15 0 g (A-2) (B-2) (C-2) (D-10) Ex. 23 32.5 19.5 13 65 35 — 0.18 0 g (A-2) (B-2) (C-2) (D-11) Ex. 24 32.5 19.5 13 65 34 1 (E-1) 0.06 0 g (A-2) (B-2) (C-2) (D-1)

TABLE 6 Silane Thermally conductive coupling Thermal material (vol %) Base oil agent resistance Content of each material Total (vol %) (vol %) (° C./W) Separation Comp. 32.5 19.5 13 65 35 — 0.83 0 g Ex. 1 (A-4) (B-2) (C-2) (D-1) Comp. 32.5 19.5 13 85 35 — 0.73 0 g Ex. 2 (A-5) (B-2) (C-2) (D-1) Comp. 32.5 19.5 13 65 35 — 0.55 0 g Ex. 3 (A-2) (B-4) (C-2) (D-1) Comp. 32.5 19.5 13 65 35 — 0.25 0 g Ex. 4 (A-2) (B-5) (C-2) (D-1) Comp. 32.5 19.5 13 65 35 — 0.73 0 g Ex. 5 (A-2) (B-2) (C-4) (D-1) Comp. 32.5 19.5 13 65 35 — 0.55 0 g Ex. 6 (A-2) (B-2) (C-5) (D-1) Comp. 32.5 19.5 13 65 35 — 0.73 0 g Ex. 7 (A-2) (B-2) (C-2) (D-5) Comp. 32.5 19.5 13 65 35 — 0.55 0.2 g   Ex. 8 (A-2) (B-2) (C-2) (D-6)

The grease of the present invention shows low thermal resistance, it is not significantly deteriorated by heat cycle, and thus, it can efficiently transfer heat from heat-generating electronic components to a cooling portion such as a heat sink or a casing.

INDUSTRIAL APPLICABILITY

The thermally conductive grease of the present invention can be suitably applied to various applications. In particular, since the grease can efficiently transfer heat when the grease is disposed between a heat-generating electronic components and e.g. a heat sink, the grease is usable for e.g. cooling of heat-generating electronic components.

The entire disclosure of Japanese Patent Application No. 2006-282457 filed on Oct. 17, 2006 including specification, claims and summary is incorporated herein by reference in its entirety. 

1. A grease comprising at least one thermally conductive material powder selected from the group consisting of a thermally conductive material (A), a thermally conductive material (B) and a thermally conductive material (C), and a base oil having a surface tension of from 25 to 40 dyn/cm at 25° C., wherein the particle size distribution of the thermally conductive material powder measured by a laser diffraction type particle size distribution method, has population peaks in the ranges of from 2.0 to 10 μm, from 1.0 to 1.9 μm and from 0.1 to 0.9 μm, respectively.
 2. A grease comprising a thermally conductive material (A) having an average particle size of from 2.0 to 10 μm, a thermally conductive material (B) having an average particle size of from 1.0 to 1.9 μm, a thermally conductive material (C) having an average particle size of from 0.1 to 0.9 μm, and a base oil having a surface tension of from 25 to 40 dyn/cm at 25° C.
 3. The grease according to claim 1, wherein each of the thermally conductive material (A), (B) and (C) is at least one member selected from the group consisting of metal aluminum, aluminum nitride and zinc oxide.
 4. The grease according to claim 1, wherein the thermally conductive material (A) is metal aluminum, the thermally conductive material (B) is aluminum nitride, and the thermally conductive material (C) is zinc oxide.
 5. The grease according to claim 1, wherein the viscosity of the base oil is from 300 to 1,000 mPa·s.
 6. The grease according to claim 1, wherein the base oil is a silicone oil having an alkyl group.
 7. The grease according to claim 1, wherein the content of the thermally conducive materials (A), (B) and (C) is from 60 to 80 vol %.
 8. The grease according to claim 1, wherein based on the total thermally conductive material, the thermally conductive material (A) is from 50 to 70 vol %, the thermally conductive material (B) is from 30 to 20 vol % and the thermally conductive material (C) is from 20 to 10 vol %.
 9. The grease according to claim 1, which further comprises a silane coupling agent.
 10. The grease according to claim 1, which has a thermal resistance of at most 0.2° C./W.
 11. The grease according to claim 2, wherein the thermally conductive material (A), (B) or (C) is at least one member selected from the group consisting of metal aluminum, aluminum nitride and zinc oxide.
 12. The grease according to claim 2, wherein the thermally conductive material (A) is metal aluminum, the thermally conductive material (B) is aluminum nitride, and the thermally conductive material (C) is zinc oxide.
 13. The grease according to claim 2, wherein the viscosity of the base oil is from 300 to 1,000 mPa·s.
 14. The grease according to claim 3, wherein the viscosity of the base oil is from 300 to 1,000 mPa·s.
 15. The grease according to claim 11, wherein the viscosity of the base oil is from 300 to 1,000 mPa·s.
 16. The grease according to claim 4, wherein the viscosity of the base oil is from 300 to 1,000 mPa·s.
 17. The grease according to claim 12, wherein the viscosity of the base oil is from 300 to 1,000 mPa·s.
 18. The grease according to claim 2, wherein the base oil is a silicone oil having an alkyl group.
 19. The grease according to claim 2, wherein the content of the thermally conducive materials (A), (B) and (C) is from 60 to 80 vol %.
 20. The grease according to claim 2, wherein based on the total thermally conductive material, the thermally conductive material (A) is from 50 to 70 vol %, the thermally conductive material (B) is from 30 to 20 vol % and the thermally conductive material (C) is from 20 to 10 vol %.
 21. The grease according to claim 2, which further comprises a silane coupling agent.
 22. The grease according to claim 2, which has a thermal resistance of at most 0.2° C./W. 